The present invention relates to the field of automobile safety and in particular the protection, in the event of an impact, of the passengers of a motor vehicle by inflation of a protective pillow by means of the combustion gases originating from a cold-gas pyrotechnic generator. To be more precise, the invention relates to a cold-gas generator of this type with welded coaxial chambers and a method of manufacture applicable to a generator of this type enabling it to be loaded with pyrotechnic material after the mechanical components forming the various chambers have been welded.
It has been envisaged for many years now to protect the passengers of a motor vehicle, in the event of an impact, by inflation of a pillow which comes between the body of the passenger and the dashboard of the vehicle. Inflation of the pillow is ensured by a gas source released by an acceleration or deceleration detector calibrated above a certain threshold. In order to ensure the reliability of the protective device over time, the gas source must both preserve well over time and have a very short response time, of the order of only a few tens of milliseconds.
These two requirements have led a person skilled in the art to prefer as the gas source cold-gas pyrotechnic generators in the form of metal casings which consist essentially of a base and a cover defining inside the casing an ignition chamber which contains an ignition device connected to the acceleration or deceleration detector, a combustion chamber which contains a gas-generating pyrotechnic composition, and one or more chambers for filtering or cooling the gases. Since these gas generators must have a reduced size, and in particular a reduced height, so as to be able to be housed in particular in steering wheels of motor vehicles, a person skilled in the art generally adopts a cylindrical shape for these generators, the ignition chamber being central and the combustion and filtering chambers having an annular shape and being arranged either inside each other or on top of each other, the combustion chamber surrounding all or part of the ignition chamber. Generators of this type are, for example, described in U.S. Pats. Nos. 4,547,342, 4,578,247 and 4,590,041.
The pyrotechnic composition employed is generally a mixture of sodium nitride and mineral oxidizing agents. Since sodium nitride is an explosive compound sensitive to humidity, it is recommended that the gas generator be sealed tightly and so that it cannot be removed, on the one hand in order to protect the composition from humidity and on the other hand to prevent any inopportune feedback of the explosive component. For these reasons, the base and the cover forming the casing of the generator are usually welded together, it being possible for welding to be carried out advantageously by friction or by inertia, as indicated in the patents cited hereinabove.
However, this solution has a serious disadvantage. Most of the designs adopted up until now for gas generators do not allow them to be filled with pyrotechnic material after welding and it is thus necessary to fill them with pyrotechnic material before welding and to carry out this operation in the presence of pyrotechnic material which contains a high degree of sodium nitride, an operation which is far from being free of danger.
It is known to the Applicant that an attempt has been made to make possible this type of filling. This attempt is described in U.S. Pat. No. 4,249,673. This patent describes a gas generator of revolution formed by a base and a cover establishing two concentric chambers, an inner ignition and combustion chamber and a peripheral annular cooling and filtering chamber. The base of the generator has a central orifice to which it is possible, after the generator has been assembled, to introduce pellets of a pyrotechnic composition, and then the ignition device, into the inner chamber. Although the solution described in this patent allows the filling of the generator with pyrotechnic material after the base and the cover have been assembled, it nevertheless has two other serious disadvantages. So as to be able to ensure filling of the pellets and introduction of the ignition device, the generator must be given a height which is very much greater than that of the igniter. This makes it impossible to meet the standards imposed by constructors of motor vehicles who demand "flat" generators, whose height barely exceeds that of the igniters. Moreover, after loading, the pellets of a pyrotechnic composition are arranged around the igniter and above the latter, which makes it impossible to ensure the symmetry of the ignition and the run-off of the gases into the generator where symmetry is necessary if very short response times are desired without risking an over-pressure in the generator.
At the present time a person skilled in the art does not therefore have at his disposal a gas generator having good symmetry of ignition and run-off of the gases and capable of being loaded safely with pyrotechnic material after the constituent elements of its casing have been welded.